1. Field of the Invention
The present invention relates generally to plasticizers for melting solid plastic feed material prior to molding, blowing and the like. More particularly, it relates to a plasticizer having a chamber with opposed walls which are resonantly reciprocated to melt the plastic feed material by the application of pressure.
2. Description of the Prior Art
The most common method of melting solid plastic material prior to injection molding, blowing plastic film and the like is to pass solid plastic pellets or granules through a screw extruder where the plastic is melted by the combined friction and compression of the extruder. Often, heating elements are provided to complete the melting.
A more recent method is described in an article entitled "The Teledynamic Injection Molding Machine, A Non-Screw Injection Molding System" by Rossig and Walheim, KURSTOFFE 62, 541-43, Sept. 1972 and is referred to as high pressure plasticization. Solid plastic particles are advanced through a plasticizer or extruder by a plunger that is pulsating at a frequency of from 4 to 5 cycles per second while simultaneously applying a basal pressure of approximately 45,000 psi. High pressure plasticization may be carried out at a lower internal mass temperature and over a shorter time period than conventional screw injection plasticization avoiding thermal degradation of the mold mass and facilitating material flow through flow paths. Injection molded parts can be produced with less shrinkage and less strain.
Despite its advantages, high pressure plasticization has not been commercially successful. One reason for the lack of commercial success has been the general inability of the plasticizing machine, as described in the aforementioned article, to withstand the severe stress placed on its components by the large forces requires to produce the high pressure. The machine disclosed comprises plasticizing a chamber having a relatively small volume (in the range from 200 to 1,000 cc) and a plunger designed to reciprocate in the chamber. The plunger is driven directly by an eccentric drive unit which superimposes the pressure pulse over the basal pressure. The reaction forces and stresses generated by the drive system often damage the machine. It is thus desirable to provide a high pressure plasticizer which is relatively vibration free and which avoids the stress-associated problems of the prior art.
An additional shortcoming of the prior art is the limited volume available for the plasticizing chamber. The limited volume results from the need to maintain a relatively high ratio of the area to which pressure is applied to volume. Since it is also required to have a relatively small plunger area to apply the very high pressure spikes required for plasticization, it is necessary to limit the volume of the chamber. It is thus also desirable that a high pressure plasticizer having a relatively large plasticizing chamber be provided.